1. Field of the Invention
The present invention relates to inertial multisensor navigation units (IMU's) for short range, relatively low-accuracy guidance applications, such as munitions. More particularly, this invention pertains to a flexure for use in a multisensor of the type that employs paired triads of accelerometers mounted on counter-oscillating platforms for directly measuring linear accelerations and for determining rotation rates with respect to a three-axis system from Coriolis accelerations.
2. Description of the Prior Art
Multisensors measure space-dependent accelerations and rotation, or angular, rates with respect to orthogonal space axes. Their design is beset by numerous complexities, as it requires the simultaneous measurement of six independent variables. For example, gyroscopes of the ring laser and fiber optic type require a lasing cavity dedicated to each input axis. This mandates a total of three lasing cavities, an expensive undertaking, to obtain three of the six required measurements. (An example of a laser device for measuring rotation about three axes is shown in U.S. Pat. No. 4,795,258 of Graham Martin, property of the assignee herein, entitled "Nonplanar Three-Axis Ring Laser Gyro With Shared Mirror Faces".) Multisensors employing spinning wheel gyros must deal with their limitation to measurement of rotation with respect to two axes, necessitating the use of an additional drive and servo or capture mechanisms for a third and fourth (redundant) input axis. Again, this does not in any way account for the additional complexity introduced by the remaining measurements of accelerations.
Simplicity and economy are particularly significant in the design of multisensors for munitions guidance and like applications. Such uses are characterized by non-reusable payloads, limited flight durations and only moderate accuracy requirements. One economical type of system for measuring both rotation rates and linear acceleration with reference to a set of three orthogonal axes is a multi-sensor mechanism taught in a series of United States patents, also the property of the assignee herein: (Ser. Nos. 4,996,877, entitled, "Three Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillator"; 5,007,289, entitled, "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator"; and 5,065,627 entitled, "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator".) The teachings of these patents are incorporated herein by this reference. The devices disclosed in the above-referenced patents employ piezoelectric drive mechanisms to drive a pair of counterbalanced platforms to oscillate out-of-phase about a common axis within a housing or case. Accelerometers, housed in a vacuum to avoid the effects of gas damping, are mounted at tilted attitudes (for measuring variables in orthogonal planes) with respect to radially-directed elements of the platforms to provide measures of both linear acceleration and rotation. The latter (rotation) values are derived from the (Coriolis) forces sensed by the accelerometers at the resonant frequency of the counter-oscillating structure.
The oscillatory motions of the rotors of the multisensors taught by the above-identified patents are coupled to one another through the case that houses the mechanism. Each rotor comprises three radially-directed rotor arms. An accelerometer is fixed to each rotor arm. The rotor arms alternate with rotor platforms, each including three radially-directed webs. Piezoelectric elements are mounted to each side of the two outer webs. The elements are appropriately-poled so that an input drive signal simultaneously induces compression and tension at the opposite surfaces to cause predetermined bending of the webs to produce oscillation of the rotors. The central web is relatively stiff, providing the major factor for determining the natural or resonant frequency of the rotor.
Each rotor is bolted exclusively to the case for support, whereby the case provides the sole path of energy transfer between the oscillating rotors. As mentioned earlier, measurement of rotation rate through sensing Coriolis acceleration relies upon the demodulation of an output signal whose frequency is equal to the resonant frequency of the paired rotors, with a single resonant frequency assumed. The above-described design is subject to factors that can complicate the measurement of rotation rate to a significant extent. Some complications follow from the only-indirect coupling of energy (i.e., through the case) between the paired rotors.
Numerous arrangements may act to weaken the already-indirect coupling of energy. For example, many multisensor applications require hard-mounting of the case to a body. In such arrangements, the mechanical impedance of the outside world is introduced into the rotor coupling so that the transfer of energy between the oscillating rotors is subject to attenuation in complex, and sometimes-unforeseen, ways. Thus, the accuracy of rotation rate measurement can vary as a function of application and changes in mechanical impedance.
Solutions to problems relating to such weakly-coupled rotors, for overcoming energy leakage as well as problems related to differential rotor frequencies are quite complex and often expensive to implement. One solution, adjusting the relative amplitudes of the rotor drive voltages, can introduce bias effects, complicate system electronics, etc. Another solution is to mount the multi-sensor case on isolators so that the device is no longer hard-mounted to the outside world. While essentially solving the problems of external impedances, isolation-mounting multiplies mechanical complexity, size and cost, often to a significant extent.
Pending U.S. patent application Ser. No. 08/904,927, also property of the assignee herein, entitled "Multisensor with Directly Coupled Rotors," addresses the above-described problems of multisensor arrangements in which oscillatory energy is coupled through the case by providing a multisensor in which the opposite ends of a shaft-like torsion spring are fixed to the centers of rotation of the aligned rotors. The axis of the torsion spring thereby lies coincident with the common axis of rotation of the rotor pair.
While offering the advantage of direct transfer of energy from rotor to rotor, the fabrication of such a device is difficult. Precise alignment of the axis of the shaft-like torsion spring is necessary to assure that the two rotors counter-oscillate in parallel planes. This can be a particularly difficult task, as the rotors are mounted not only to each other through the torsion spring, but are also mounted to the case through associated flexures. Should the rotors joined by the common torsion spring be even slightly out of alignment, tension and compression forces thereby introduced into those flexures will affect the resonant frequency of the device and act as a source of bias error. Very high precision, complex assembly processes and skilled technicians are required to install the rotor-and-flexure arrangement, further increasing cost.